The Queen met adoring fans and opened a bandstand in the first of the events to mark her 90th birthday.
On the eve of the big day, the Queen, accompanied by The Duke of Edinburgh, greeted children on her way to watch a choir at the bandstand in Alexandria Gardens in Windsor.
The monarch also visited the Royal Mail delivery office to mark the 500th Anniversary of the Postal Service.
Ten special stamps have been released to celebrate the Queen's birthday, including a stamp sheet featuring four generations of the Royal Family.
The Queen met long-serving postmen and women, viewed the operation of the Caller's Office and unveiled a plaque marking the visit.
Tomorrow the Queen will unveil a plaque on Windsor's Queen's Walkway, before lighting the first of a chain of beacons. Gun salutes will also be held across the UK to mark her 90th birthday.
Her birthday celebrations will culminate on June 12 with a street party for 10,000 people in the Mall.